OSLO (Reuters) - Sea levels could rise 50 cm (20 inches) more this century than had been expected, according to a report published on Wednesday which found that Antarctic ice will melt faster than previously thought.

Climate scientists at two U.S. universities said the most recent U.N. report on the effects of global warming had underestimated the rate at which the ice covering the continent would melt.

That report, issued in 2013, said the worst case of man-made climate change would mean a sea-level rise of between 52 and 98 cm by 2100. The new study suggests the real rise could be 1.5 meters (5 ft), posing an even greater threat to cities from New York to Shanghai.

"This could spell disaster for many low-lying cities," lead author Robert DeConto at the University of Massachusetts, Amherst, said in a statement of the findings published in the journal Nature.

"For example, Boston could see more than 1.5 meters of sea-level rise in the next 100 years."

The study, partly based on sea level evidence in a natural warm period 125,000 years ago, said ice from Antarctica alone could cause between 64 cm and 114 cm of sea level rise by 2100 under the worst U.N. scenario for greenhouse gas emissions.

One of the factors that has was underestimated in the U.N. reports, which envisage most Antarctic ice remaining frozen, is a process known as "hydro fracturing" whereby pools of meltwater on ice shelves seep deep into the ice, refreeze and force vast chunks of ice to crack off. That could make ice on land in Antarctica slide faster into the sea.

Anders Levermann, an expert at the Potsdam Institute for Climate Impact Research who was not involved in the study, welcomed the findings as a spur to research hydro fracturing in Antarctica.

Several other studies have highlighted risks of rising seas. Former NASA scientist James Hansen suggested on March 22 that there could be "several meters" of sea level rise in the coming century.

But Levermann dismissed Hansen's findings.

"It's plain wrong," he told Reuters of Hansen's assumption that the rate of sea level rise could repeatedly double in coming decades.

Wednesday's study projected that Antarctica could contribute more than 13 meters of sea level rise by 2500 if the air and oceans keep warming.

A global agreement to cut greenhouse gas emissions, reached at a 195-nation summit in Paris in December, is meant to help limit the rise is sea levels and other potentially disastrous effects of climate change.

But even if the cuts are fully implemented, they will not completely halt global warming, scientists say. Seas have risen about 20 cm since 1900, according to U.N. studies.